Composite polyamide membrane made via interfacial polymerization using a blend of non-polar solvents

ABSTRACT

A method for making a composite polyamide membrane comprising a porous support and a thin film polyamide layer, wherein the method includes: i) applying a polar solution comprising a polyfunctional amine monomer, and a non-polar solution comprising a polyfunctional acyl halide monomer to a surface of a porous support and interfacially polymerizing the monomers to form a thin film polyamide layer, wherein the non-polar solution further comprises at least 50 vol % of a C 5  to C 20  aliphatic hydrocarbon and from 2 to 25 vol % of benzene or benzene substituted with one or more C 1  to C 6  alkyl groups; and ii) applying an aqueous solution of nitrous acid to the thin film polyamide layer.

FIELD

The present invention is generally directed toward composite polyamide membranes along with methods for making and using the same.

INTRODUCTION

Composite polyamide membranes are used in a variety of fluid separations. One common class of membranes includes a porous support coated with a “thin film” polyamide layer. The thin film layer may be formed by an interfacial polycondensation reaction between polyfunctional amine (e.g. m-phenylenediamine) and polyfunctional acyl halide (e.g. trimesoyl chloride) monomers which are sequentially coated upon the support from immiscible solutions, see for example U.S. Pat. No. 4,277,344 to Cadotte. Various constituents may be added to one or both of the coating solutions to improve membrane performance. For example, U.S. Pat. No. 4,259,183 to Cadotte describes the use of combinations of bi- and tri-functional acyl halide monomers, e.g. isophthaloyl chloride or terephthaloyl chloride with trimesoyl chloride. U.S. Pat. No. 6,878,278 to Mickols describes the addition of a wide range of complexing agents to the acyl halide coating solution, including various tri-hydrocarbyl phosphate compounds. US 2011/0049055 describes the addition of moieties derived from sulfonyl, sulfinyl, sulfenyl, sulfuryl, phosphoryl, phosphonyl, phosphinyl, thiophosphoryl, thiophosphonyl and carbonyl halides. U.S. Pat. No. 6,521,130 describes the addition of a carboxylic acid (e.g. aliphatic and aromatic carboxylic acids) or carboxylic acid ester to one or both monomer coating solutions prior to polymerization. Similarly, U.S. Pat. Nos. 6,024,873, 5,989,426, 5,843,351 and 5,576,057 describe the addition of selected alcohols, ethers, ketones, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds having solubility parameters of 8 to 14 (cal/cm³)^(1/2) to one of the coating solutions. US 2009/0107922 describes the addition of various “chain capping reagents” to one or both coating solutions, e.g. 1,3 propane sultone, benzoyl chloride, 1,2-bis(bromoacetoxy) ethane, etc. U.S. Pat. Nos. 4,606,943 and 6,406,626 describe the formation of a thin film polyamide using a polyfunctional amine and polyfunctional acyl halide along with a polyfunctional acid anhydride halide (e.g. trimellitic anhydride acyl chloride). WO2012/102942, WO2012/102943 and WO2012/102944 describe the addition of various monomers including carboxylic acid and amine-reactive functional groups. US 2009/0272692, US 2012/0261344 and U.S. Pat. No. 8,177,978 describe the use of various polyfunctional acyl halides and their corresponding partially hydrolyzed counterparts. U.S. Pat. Nos. 4,812,270 and 4,888,116 to Cadotte (see also WO 2012/090862, WO 2012/020680, WO 2011/105278, WO 2011/078131 and WO 2011/078047) describe post-treating the membrane with phosphoric or nitrous acid. U.S. Pat. No. 5,582,725 describes a similar post treatment with an acyl halide such as benzoyl chloride. The search continues for new combinations of monomers, additives and post-treatments that further improve membrane performance.

SUMMARY

The invention includes a method for making a composite polyamide membrane including a porous support and a thin film polyamide layer. The method includes the steps of:

i) applying a polar solution comprising a polyfunctional amine monomer, and a non-polar solution comprising a polyfunctional acyl halide monomer to a surface of a porous support and interfacially polymerizing the monomers to form a thin film polyamide layer,

wherein the non-polar solution further comprises at least 50 vol % of a C₅ to C₂₀ aliphatic hydrocarbon and from 2 to 25 vol % of benzene or benzene substituted with one or more C₁ to C₆ alkyl groups; and

ii) applying an aqueous solution of nitrous acid to the thin film polyamide layer.

Many additional embodiments are described including applications for such membranes.

DETAILED DESCRIPTION

The invention is not particularly limited to a specific type, construction or shape of composite membrane or application. For example, the present invention is applicable to flat sheet, tubular and hollow fiber polyamide membranes useful in a variety of applications including forward osmosis (FO), reverse osmosis (RO), nano filtration (NF), ultra filtration (UF), micro filtration (MF) and pressure retarded fluid separations. However, the invention is particularly useful for membranes designed for RO and NF separations. RO composite membranes are relatively impermeable to virtually all dissolved salts and typically reject more than about 95% of salts having monovalent ions such as sodium chloride. RO composite membranes also typically reject more than about 95% of inorganic molecules as well as organic molecules with molecular weights greater than approximately 100 Daltons. NF composite membranes are more permeable than RO composite membranes and typically reject less than about 95% of salts having monovalent ions while rejecting more than about 50% (and often more than 90%) of salts having divalent ions—depending upon the species of divalent ion. NF composite membranes also typically reject particles in the nanometer range as well as organic molecules having molecular weights greater than approximately 200 to 500 Daltons.

Examples of composite polyamide membranes include FilmTec Corporation FT-30™ type membranes, i.e. a flat sheet composite membrane comprising a bottom layer (back side) of a nonwoven backing web (e.g. PET scrim), a middle layer of a porous support having a typical thickness of about 25-125 μm and top layer (front side) comprising a thin film polyamide layer having a thickness typically less than about 1 micron, e.g. from 0.01 micron to 1 micron but more commonly from about 0.01 to 0.1 μm. The porous support is typically a polymeric material having pore sizes which are of sufficient size to permit essentially unrestricted passage of permeate but not large enough so as to interfere with the bridging over of a thin film polyamide layer formed thereon. For example, the pore size of the support preferably ranges from about 0.001 to 0.5 μm. Non-limiting examples of porous supports include those made of: polysulfone, polyether sulfone, polyimide, polyamide, polyetherimide, polyacrylonitrile, poly(methyl methacrylate), polyethylene, polypropylene, and various halogenated polymers such as polyvinylidene fluoride. For RO and NF applications, the porous support provides strength but offers little resistance to fluid flow due to its relatively high porosity.

Due to its relative thinness, the polyamide layer is often described in terms of its coating coverage or loading upon the porous support, e.g. from about 2 to 5000 mg of polyamide per square meter surface area of porous support and more preferably from about 50 to 500 mg/m². The polyamide layer is preferably prepared by an interfacial polycondensation reaction between a polyfunctional amine monomer and a polyfunctional acyl halide monomer upon the surface of the porous support as described in U.S. Pat. Nos. 4,277,344 and 6,878,278. More specifically, the polyamide membrane layer may be prepared by interfacially polymerizing a polyfunctional amine monomer with a polyfunctional acyl halide monomer, (wherein each term is intended to refer both to the use of a single species or multiple species), on at least one surface of a porous support. As used herein, the term “polyamide” refers to a polymer in which amide linkages (—C(O)NH—) occur along the molecular chain. The polyfunctional amine and polyfunctional acyl halide monomers are most commonly applied to the porous support by way of a coating step from solution, wherein the polyfunctional amine monomer is typically coated from an aqueous-based or polar solution and the polyfunctional acyl halide from an organic-based or non-polar solution. Although the coating steps need not follow a specific order, the polyfunctional amine monomer is preferably first coated on the porous support followed by the polyfunctional acyl halide. Coating can be accomplished by spraying, film coating, rolling, or through the use of a dip tank among other coating techniques. Excess solution may be removed from the support by air knife, dryers, ovens and the like.

The polyfunctional amine monomer comprises at least two primary amine groups and may be aromatic (e.g., m-phenylenediamine (mPD), p-phenylenediamine, 1,3,5-triaminobenzene, 1,3,4-triaminobenzene, 3,5-diaminobenzoic acid, 2,4-diaminotoluene, 2,4-diaminoanisole, and xylylenediamine) or aliphatic (e.g., ethylenediamine, propylenediamine, cyclohexanne-1,3-diameine and tris (2-diaminoethyl) amine). One particularly preferred polyfunctional amine is m-phenylene diamine (mPD). The polyfunctional amine monomer may be applied to the porous support as a polar solution. The polar solution may contain from about 0.1 to about 10 wt % and more preferably from about 1 to about 6 wt % polyfunctional amine monomer. In one set of embodiments, the polar solutions includes at least 2.5 wt % (e.g. 2.5 to 6 wt %) of the polyfunctional amine monomer. Once coated on the porous support, excess solution may be optionally removed.

The polyfunctional acyl halide monomer comprises at least two acyl halide groups and preferably no carboxylic acid functional groups and may be coated from a non-polar solvent although the polyfunctional acyl halide may be alternatively delivered from a vapor phase (e.g., for polyfunctional acyl halides having sufficient vapor pressure). The polyfunctional acyl halide is not particularly limited and aromatic or alicyclic polyfunctional acyl halides can be used along with combinations thereof. Non-limiting examples of aromatic polyfunctional acyl halides include: trimesoyl chloride, terephthalic acyl chloride, isophthalic acyl chloride, biphenyl dicarboxylic acyl chloride, and naphthalene dicarboxylic acid dichloride. Non-limiting examples of alicyclic polyfunctional acyl halides include: cyclopropane tri carboxylic acyl chloride, cyclobutane tetra carboxylic acyl chloride, cyclopentane tri carboxylic acyl chloride, cyclopentane tetra carboxylic acyl chloride, cyclohexane tri carboxylic acyl chloride, tetrahydrofuran tetra carboxylic acyl chloride, cyclopentane dicarboxylic acyl chloride, cyclobutane dicarboxylic acyl chloride, cyclohexane dicarboxylic acyl chloride, and tetrahydrofuran dicarboxylic acyl chloride. One preferred polyfunctional acyl halide is trimesoyl chloride (TMC). The polyfunctional acyl halide may be dissolved in a non-polar solvent in a range from about 0.01 to 10 wt %, preferably 0.05 to 3% wt % and may be delivered as part of a continuous coating operation. In one set of embodiments wherein the polyfunctional amine monomer concentration is less than 3 wt %, the polyfunctional acyl halide is less than 0.3 wt %.

The non-polar solution comprises a blend of aliphatic and aromatic solvents. More specifically, the non-polar solution comprises from 2 to 25 vol % (and more preferably from 3 to 15 vol %) of benzene or benzene substituted with one or more C₁-C₆ alkyl groups as represented in Formula (I).

wherein R₁ through R₆ independently selected from hydrogen or an alkyl group having from 1 to 6 carbon atoms. In a preferred set of embodiments, the benzene ring is substituted with a plurality of methyl groups, e.g. 1,3,5-trimethylbenzene (“mesitylene”). The non-polar solution further comprises at least 50 vol % (e.g. from 50 to 98 vol % and more preferably from 50 to 90 vol %) of a C₅ to C₂₀ aliphatic hydrocarbon solvent. Representative examples include paraffins (e.g. hexane, cyclohexane, heptane, octane, dodecane) and isoparaffins (e.g. ISOPAR™ L).

In a preferred subset of embodiments, the non-polar solution further comprises an acid-containing monomer comprising a C₂-C₂₀ hydrocarbon moiety substituted with at least one carboxylic acid functional group or salt thereof and at least one amine-reactive functional group selected from: acyl halide, sulfonyl halide and anhydride, wherein the acid-containing monomer is distinct from the polyfunctional acyl halide monomer. In one set of embodiments, the acid-containing monomer comprises an arene moiety. Non-limiting examples include mono and di-hydrolyzed counterparts of the aforementioned polyfunctional acyl halide monomers including two to three acyl halide groups and mono, di and tri-hydrolyzed counterparts of the polyfunctional halide monomers that include at least four amine-reactive moieties. A preferred species includes 3,5-bis(chlorocarbonyl)benzoic acid (i.e. mono-hydrolyzed trimesoyl chloride or “mhTMC”). Additional examples of monomers are described in WO 2012/102942 and WO 2012/102943 (see Formula III wherein the amine-reactive groups (“Z”) are selected from acyl halide, sulfonyl halide and anhydride). Specific species including an arene moiety and a single amine-reactive group include: 3-carboxylbenzoyl chloride, 4-carboxylbenzoyl chloride, 4-carboxy phthalic anhydride and 5-carboxy phthalic anhydride, and salts thereof. Additional examples are represented by Formula (II):

wherein A is selected from: oxygen (e.g. —O—); amino (—N(R)—) wherein R is selected from a hydrocarbon group having from 1 to 6 carbon atoms, e.g. aryl, cycloalkyl, alkyl—substituted or unsubstituted but preferably alkyl having from 1 to 3 carbon atoms with or without substituents such as halogen and carboxyl groups); amide (—C(O)N(R))— with either the carbon or nitrogen connected to the aromatic ring and wherein R is as previously defined; carbonyl (—C(O)—); sulfonyl (—SO₂—); or is not present (e.g. as represented in Formula III); n is an integer from 1 to 6, or the entire group is an aryl group; Z is an amine reactive functional group selected from: acyl halide, sulfonyl halide and anhydride (preferably acyl halide); Z′ is selected from the functional groups described by Z along with hydrogen and carboxylic acid. Z and Z′ may be independently positioned meta or ortho to the A substituent on the ring. In one set of embodiments, n is 1 or 2. In yet another set of embodiments, both Z and Z′ are both the same (e.g. both acyl halide groups). In another set of embodiments, A is selected from alkyl and alkoxy groups having from 1 to 3 carbon atoms. Non-limiting representative species include: 2-(3,5-bis(chlorocarbonyl)phenoxy)acetic acid, 3-(3,5-bis(chlorocarbonyl)phenyl) propanoic acid, 2-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)oxy)acetic acid, 3-(1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)propanoic acid, 2-(3-(chlorocarbonyl) phenoxy)acetic acid, 3-(3-(chlorocarbonyl)phenyl)propanoic acid, 3-((3,5bis(chlorocarbonyl)phenyl) sulfonyl) propanoic acid, 3-((3-(chlorocarbonyl)phenyl)sulfonyl)propanoic acid, 3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)sulfonyl)propanoic acid, 3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)amino) propanoic acid, 3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)(ethyl)amino)propanoic acid, 3-((3,5-bis(chlorocarbonyl) phenyl)amino) propanoic acid, 3-((3,5-bis(chlorocarbonyl) phenyl)(ethyl)amino) propanoic acid, 4-(4-(chlorocarbonyl)phenyl)-4-oxobutanoic acid, 4-(3,5-bis(chlorocarbonyl)phenyl)-4-oxobutanoic acid, 4-(1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)-4-oxobutanoic acid, 2-(3,5-bis(chlorocarbonyl) phenyl)acetic acid, 2-(2,4-bis(chlorocarbonyl)phenoxy) acetic acid, 4-((3,5-bis(chlorocarbonyl) phenyl)amino)-4-oxobutanoic acid, 2-((3,5-bis(chloro carbonyl)phenyl)amino)acetic acid, 2-(N-(3,5-bis(chlorocarbonyl)phenyl)acetamido)acetic acid, 2,2′-((3,5-bis(chlorocarbonyl)phenylazanediyl) diacetic acid, N-[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]-glycine, 4-[[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]amino]-benzoic acid, 1,3-dihydro-1,3-dioxo-4-isobenzofuran propanoic acid, 5-[[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]amino]-1,3-benzenedicarboxylic acid and 3-[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)sulfonyl]-benzoic acid.

Another embodiment is represented by Formula (III).

wherein the carboxylic acid group may be located meta, para or ortho upon the phenyl ring.

Representative examples where the hydrocarbon moiety is an aliphatic group are represented by Formula (IV):

wherein X is a halogen (preferably chlorine) and n is an integer from 1 to 20, preferably 2 to 10. Representative species include: 4-(chlorocarbonyl) butanoic acid, 5-(chlorocarbonyl) pentanoic acid, 6-(chlorocarbonyl) hexanoic acid, 7-(chlorocarbonyl) heptanoic acid, 8-(chlorocarbonyl) octanoic acid, 9-(chlorocarbonyl) nonanoic acid, 10-(chlorocarbonyl) decanoic acid, 11-chloro-11-oxoundecanoic acid, 12-chloro-12-oxododecanoic acid, 3-(chlorocarbonyl)cyclobutanecarboxylic acid, 3-(chlorocarbonyl)cyclopentane carboxylic acid, 2,4-bis(chlorocarbonyl)cyclopentane carboxylic acid, 3,5-bis(chlorocarbonyl) cyclohexanecarboxylic acid, and 4-(chlorocarbonyl) cyclohexanecarboxylic acid. While the acyl halide and carboxylic acid groups are shown in terminal positions, one or both may be located at alternative positions along the aliphatic chain. While not shown in Formula (IV), the acid-containing monomer may include additional carboxylic acid and acyl halide groups.

Representative examples of acid-containing monomers include at least one anhydride group and at least one carboxylic acid groups include: 3,5-bis(((butoxycarbonyl)oxy)carbonyl)benzoic acid, 1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylic acid, 3-(((butoxycarbonyl)oxy)carbonyl)benzoic acid, and 4-(((butoxycarbonyl)oxy)carbonyl)benzoic acid.

The upper concentration range of acid-containing monomer may be limited by its solubility within the non-polar solution. In most embodiments, the upper concentration limit is less than 1 wt %. In one set of embodiments, the acid-containing monomer is provided in the non-polar solution at concentration of at least 0.01 wt %, 0.02 wt %, 0.03 wt %, 0.04 wt %, 0.05 wt %, 0.06 wt %, 0.07 wt %, 0.08 wt %, 0.1 wt % or even 0.13 wt % while remaining soluble in solution. In another set of embodiments, the non-polar solution comprises from 0.01 to 1 wt %, 0.02 to 1 wt %, 0.04 to 1 wt % or 0.05 to 1 wt % of the acid-containing monomer. The inclusion of the acid-containing monomer during interfacial polymerization between the polyfunctional amine and acyl halide monomers results in a membrane having improved performance. And, unlike post hydrolysis reactions that may occur on the surface of the thin-film polyamide layer, the inclusion of the acid-containing monomer during interfacial polymerization is believed to result in a polymer structure that is beneficially modified throughout the thin-film layer.

In a preferred embodiment, the thin film polyamide layer is characterized by having a dissociated carboxylate content of at least 0.18, 0.20, 0.22, 0.3, 0.40 and in some embodiments at least 0.45 moles/kg of polyamide at pH 9.5 as measured by a Rutherford Backscattering (RBS) measurement technique. More specifically, samples membranes (1 inch×6 inch) are boiled for 30 minutes in deionized water (800 mL), then placed in a 50/50 w/w solution of methanol and water (800 mL) to soak overnight. Next, 1 inch×1 inch size sample of these membranes are immersed in a 20 mL 1×10⁻⁴ M AgNO₃ solution with pH adjusted to 9.5 for 30 minutes. Vessels containing silver ions are wrapped in tape and to limit light exposure. After soaking with the silver ion solution, the unbound silver is removed by soaking the membranes in 2 clean 20 mL aliquots of dry methanol for 5 minutes each. Finally, the membranes are allowed to dry in a nitrogen atmosphere for a minimum of 30 minutes. Membrane samples are mounted on a thermally and electrically conductive double sided tape, which was in turn mounted to a silicon wafer acting as a heat sink The tape is preferably Chromerics Thermattach T410 or a 3M copper tape. RBS measurements are obtained with a Van de Graff accelerator (High Voltage Engineering Corp., Burlington, Mass.); A 2 MeV He⁺ room temperature beam with a diameter of 3 mm at an incident angle of 22.5°, exit angle of 52.5°, scattering angle of 150°, and 40 nanoamps (nAmps) beam current. Membrane samples are mounted onto a movable sample stage which is continually moved during measurements. This movement allows ion fluence to remain under 3×10¹⁴ He⁺/cm². Analysis of the spectra obtained from RBS is carried out using SIMNRA®, a commercially available simulation program. A description of its use to derive the elemental composition from RBS analysis of RO/NF membranes is described by; Coronell, et. al. J. of Membrane Sci. 2006, 282, 71-81 and Environmental Science & Technology 2008, 42(14), 5260-5266. Data can be obtained using the SIMNRA® simulation program to fit a two layer system, a thick polysulfone layer beneath a thin polyamide layer, and fitting a three-layer system (polysulfone, polyamide, and surface coating) can use the same approach. The atom fraction composition of the two layers (polysulfone before adding the polyamide layer, and the surface of final TFC polyamide layer) is measured first by XPS to provide bounds to the fit values. As XPS cannot measure hydrogen, an H/C ratio from the proposed molecular formulas of the polymers were used, 0.667 for polysulfone and a range of 0.60-0.67 was used for polyamide. Although the polyamides titrated with silver nitrate only introduces a small amount of silver, the scattering cross section for silver is substantially higher than the other low atomic number elements (C, H, N, O, S) and the size of the peak is disproportionately large to the others despite being present at much lower concentration thus providing good sensitivity. The concentration of silver is determined using the two layer modeling approach in SIMNRA® by fixing the composition of the polysulfone and fitting the silver peak while maintaining a narrow window of composition for the polyamide layer (layer 2, ranges predetermined using XPS). From the simulation, a molar concentration for the elements in the polyamide layer (carbon, hydrogen, nitrogen, oxygen and silver) is determined. The silver concentration is a direct reflection of the carboxylate molar concentration available for binding silver at the pH of the testing conditions. The moles of carboxylic acids groups per unit area of membrane is indicative of the number of interactions seen by a species passing through the membrane, and a larger number will thus favorably impact salt passage. This value may be calculated by multiplying the measured carboxylate content by a measured thickness and by the polyamide density. Alternatively, the carboxylate number per unit area of membrane (moles/m2) may be determined more directly by methods that measure the total complexed metal within a known area. Approaches using both Uranyl acetate and toluidine blue O dye are described in: Tiraferri, et. al., Journal of Membrane Science, 2012, 389, 499-508. An approach to determine the complexed cation (sodium or potassium) content in membranes by polymer ashing is described in (Wei Xie, et al., Polymer, Volume 53, Issue 7, 22 Mar. 2012, Pages 1581-1592).

A preferred method to determine the dissociated carboxylate number at pH 9.5 per unit area of membrane for a thin film polyamide membrane is as follows. A membrane sample is boiled for 30 minutes in deionized water, then placed in a 50 wt % solution of methanol in water to soak overnight. Next, the membrane sample is immersed in a 1×10⁻⁴M AgNO₃ solution with pH adjusted to 9.5 with NaOH for 30 minutes. After soaking in the silver ion solution, the unbound silver is removed by soaking the membranes twice in dry methanol for 30 minutes. The amount of silver per unit area is preferably determined by ashing, as described by Wei, and redissolving for measurement by ICP. Preferably, the dissociated carboxylate number at pH 9.5 per square meter of membrane is greater than 6×10⁻⁵, 8×10⁻⁵, 1×10⁻⁴, 1.2×10⁻⁴, 1.5×10⁻⁴, 2×10⁻⁴, or even 3×10⁻⁴ moles/m².

In yet another preferred embodiment, the thin film layer has an isoelectric point (IEP) of less than or equal to 4.3, 4.2, 4.1, 4, 3.8, 3.6 or in some embodiments 3.5. The isoelectric point can be determined using a standard Zeta-Potential technique with a quartz cell by electrophoretic light scattering (ELS) using Desal Nano HS instrument. For example, membrane samples (2 inch×1 inch) are first boiled for 20 minutes in DI water, then rinsed well with room temperature DI water and stored at room temperature in a fresh DI solution overnight. The samples are then loaded as per reference: 2008 “User's Manual for the Delsa™ Nano Submicron Particle Size and Zeta Potential,” and the “Pre-Course Reading” for the same instrument presented by Beckmann Coulter. pH titration is completed over a range from pH 10 to pH 2 and isoelectric point is determined at the pH where the zeta potential becomes zero.

The non-polar solution may include additional constituents including co-solvents, phase transfer agents, solubilizing agents, complexing agents and acid scavengers wherein individual additives may serve multiple functions. Representative co-solvents include: benzene, toluene, xylene, mesitylene, ethyl benzene, diethylene glycol dimethyl ether, cyclohexanone, ethyl acetate, butyl carbitol™ acetate, methyl laurate and acetone. A representative acid scavenger includes N, N-diisopropylethylamine (DIEA). The non-polar solution may also include small quantities of water or other polar additives but preferably at a concentration below their solubility limit in the non-polar solution.

Once brought into contact with one another, the polyfunctional acyl halide and polyfunctional amine monomers react at their surface interface to form a polyamide layer or film. This layer, often referred to as a polyamide “discriminating layer” or “thin film layer,” provides the composite membrane with its principal means for separating solute (e.g. salts) from solvent (e.g. aqueous feed). The reaction time of the polyfunctional acyl halide and the polyfunctional amine monomer may be less than one second but contact times typically range from about 1 to 60 seconds. The removal of the excess solvent can be achieved by rinsing the membrane with water and then drying at elevated temperatures, e.g. from about 40° C. to about 120° C., although air drying at ambient temperatures may be used. However, for purposes of the present invention, the membrane is preferably not permitted to dry and is simply rinsed (e.g. dipped) with water and optionally stored in a wet state. Once formed, the polyamide layer is treated by applying an aqueous solution of nitrous acid. A variety of techniques are described in U.S. Pat. No. 4,888,116 and are incorporated herein by reference. It is believed that the nitrous acid reacts with the residual primary amine groups present in the polyamide discrimination layer to form diazonium salt groups. At least a portion of these diazonium salt groups hydrolyze to form phenol groups or azo crosslinks via diazo-coupling. Although the aqueous solution may include nitrous acid, it preferably includes reagents that form nitrous acid in situ, e.g. an alkali metal nitrite in an acid solution or nitrosyl sulfuric acid. Because nitrous acid is volatile and subject to decomposition, it is preferably formed by reaction of an alkali metal nitrite in an acidic solution in contact with the polyamide discriminating layer. Generally, if the pH of the aqueous solution is less than about 7, (preferably less than about 5), an alkali metal nitrite will react to liberate nitrous acid. Sodium nitrite reacted with hydrochloric or sulfuric acid in an aqueous solution is especially preferred for formation of nitrous acid. The aqueous solution may further include wetting agents or surfactants. The concentration of the nitrous acid in the aqueous solution is preferably from 0.01 to 1 wt %. Generally, the nitrous acid is more soluble at 5° than at 20° C. and somewhat higher concentrations of nitrous acid are operable at lower temperatures. Higher concentrations are operable so long as the membrane is not deleteriously affected and the solutions can be handled safely. In general, concentrations of nitrous acid higher than about one-half (0.5) percent are not preferred because of difficulties in handling these solutions. Preferably, the nitrous acid is present at a concentration of about 0.1 weight percent or less because of its limited solubility at atmospheric pressure. The temperature at which the membrane is contacted can vary over a wide range. Inasmuch as the nitrous acid is not particularly stable, it is generally desirable to use contact temperatures in the range from about 0° to about 30° C., with temperatures in the range from 0° to about 20° C. being preferred. Temperatures higher than this range can increase the need for ventilation or super-atmospheric pressure above the treating solution. Temperatures below the preferred range generally result in reduced reaction and diffusion rates.

The reaction between the nitrous acid and the primary amine groups occurs relatively quickly once the nitrous acid has diffused into the membrane. The time required for diffusion and the desired reaction to occur will depend upon the concentration of nitrous acid, any pre-wetting of the membrane, the concentration of primary amine groups present and the temperature at which contact occurs. Contact times may vary from a few minutes to a few days. The optimum reaction time can be readily determined empirically for a particular membrane and treatment.

One preferred application technique involves passing the aqueous nitrous acid solution over the surface of the membrane in a continuous stream. This allows the use of relatively low concentrations of nitrous acid. When the nitrous acid is depleted from the treating medium, it can be replenished and the medium recycled to the membrane surface for additional treatment. Batch treatments are also operable. The specific technique for applying aqueous nitrous acid is not particularly limited and includes spraying, film coating, rolling, or through the use of a dip tank among other application techniques. Once treated the membrane may be washed with water and stored either wet or dry prior to use. For RO and NF applications, membranes treated with nitrous acid preferably have a NaCl rejection of at least 2% when tested using an aqueous NaCl solution (250 ppm) at 25° C. and 70 psi.

A representative reaction scheme illustrating the treatment of the polyamide with nitrous acid is provided below.

Representative Diazonium Reaction Scheme:

The thin film polyamide layer may optionally include hygroscopic polymers upon at least a portion of its surface. Such polymers include polymeric surfactants, polyacrylic acid, polyvinyl alcohol, polyvinyl acetate, polyalkylene oxide compounds, poly(oxazoline) compounds, polyacrylamides and related reaction products as generally described in U.S. Pat. Nos. 6,280,853; 7,815,987; 7,918,349 and 7,905,361. In some embodiments, such polymers may be blended and/or reacted and may be coated or otherwise applied to the polyamide membrane from a common solution, or applied sequentially.

Many embodiments of the invention have been described and in some instances certain embodiments, selections, ranges, constituents, or other features have been characterized as being “preferred.” Characterizations of “preferred” features should in no way be interpreted as deeming such features as being required, essential or critical to the invention.

EXAMPLES

Sample membranes were prepared using a pilot scale membrane manufacturing line. Polysulfone supports were casts from 16.5 wt % solutions in dimethylformamide (DMF) and subsequently soaked in an aqueous solution meta-phenylene diamine (mPD). The resulting support was then pulled through a reaction table at constant speed while a thin, uniform layer of a non-polar coating solution was applied. The non-polar coating solution included a isoparaffinic solvent (ISOPAR L), a combination of trimesoyl acid chloride (TMC), and/or 1-carboxy-3,5-dichloroformyl benzene (mhTMC) in varying ratios with and without mesitylene. Excess non-polar solution was removed and the resulting composite membrane was passed through water rinse tanks and drying ovens. Sample membrane sheets were then either (i) stored in deionized water until testing; or (ii) soaked for approximately 15 minutes in a solution at 0-10 ° C. prepared by combining 0.05% w/v NaNO₂ and 0.1 w/v % HCl and thereafter rinsed and stored in deionized water until testing.

Example 1

Testing was conducted with a 2000 ppm NaCl solution at room temperature and 150 psi. Samples subjected to post-treatment with nitrous acid are designed with an asterisk (*). The results are summarized in Table 1. “SP” refers to NaCl passage. As shown by the test data, post-treatment of samples including mesitylene demonstrated an unexpected improvement in flux.

TABLE 1 Mean mPD TMC Mesitylene (AvgFlux) Mean % change in % change Sample (wt %) (wt %) (vol %) GFD (Avg SP) Flux in SP  1-1a 2.5 0.25 0 12.8 0.95% *1-1b 2.5 0.25 0 13.5 2.11% 5.3 121.1  1-1c 2.5 0.25 10 31.1 0.51% *1-1d 2.5 0.25 10 41.5 0.97% 33.3 90.7  1-2a 3.5 0.25 0 17.0 0.75% *1-2b 3.5 0.25 0 20.7 1.31% 21.6 75.6  1-2c 3.5 0.25 10 30.9 0.43% *1-2d 3.5 0.25 10 47.2 0.75% 52.7 75.1  1-3a 4.5 0.25 0 20.9 0.57% *1-3b 4.5 0.25 0 27.3 1.21% 30.3 113.5  1-3c 4.5 0.25 10 23.9 0.45% *1-3d 4.5 0.25 10 39.6 0.69% 65.6 53.9

Example 2

Testing was conducted with a 2000 ppm NaCl solution at room temperature and 150 psi. The results are summarized in Table 2. As shown by the test data, post-treatment of samples including mesitylene and an acid-containing monomer (mhTMC) had an unexpected improvement in salt passage as compared with membranes without post-treatment, or those with post treatment but without mesitylene and an acid-containing monomer.

TABLE 2 Mean mPD TMC Mesitylene mhTMC (AvgFlux) Mean % change in % change Sample (wt %) (wt %) (v0l %) (wt %) GFD (Avg SP) Flux in SP  2-1a 2.5 0.25 0 0 12.8 0.95% *2-1b 2.5 0.25 0 0 13.5 2.11% 5.3 121.1  2-1c 2.5 0.22 10 0.03 33.9 0.46% *2-1d 2.5 0.22 10 0.03 37.8 0.48% 11.5 4.3  2-2a 3.5 0.25 0 0 17.0 0.75% *2-2b 3.5 0.25 0 0 20.7 1.31% 21.6 75.6  2-2c 3.5 0.22 10 0.03 31.5 0.36% *2-2d 3.5 0.22 10 0.03 41.3 0.37% 31.1 3.7  2-3a 4.5 0.25 0 0 20.9 0.57% *2-3b 4.5 0.25 0 0 27.3 1.21% 30.3 113.5  2-3c 4.5 0.22 10 0.03 24.5 0.66% *2-3d 4.5 0.22 10 0.03 32.0 0.38% 30.8 −42.9

Example 3

Testing was conducted with a 2000 ppm NaCl solution at room temperature and 150 psi. The results are summarized in Table 3. As shown by the test data, post-treatment of samples including mesitylene and an acid-containing monomer (mhTMC) had an unexpected improvement in salt passage (SP) for membranes prepared with increasing quantities of the acid-containing monomer (mhTMC).

TABLE 3 Mean (Avg mPD TMC Mesitylene mhTMC Flux) Mean Sample (wt %) (wt %) (vol %) (wt %) GFD (Avg SP)  3a 3.5 0.25 0 0.00 30.4 0.47% *3b 3.5 0.25 0 0.00 40.6 0.61%  3c 3.5 0.24 10 0.01 32.2 0.41% *3d 3.5 0.24 10 0.01 44.9 0.50%  3e 3.5 0.20 0 0.05 33.3 0.39% *3f 3.5 0.20 0 0.05 44.7 0.40%  3g 3.5 0.17 10 0.08 34.1 0.71% *3h 3.5 0.17 10 0.08 46.8 0.46% 

The invention claimed is:
 1. A method for making a composite polyamide membrane comprising a porous support and a thin film polyamide layer, wherein the method comprises: i) applying a polar solution comprising 2.5 to 6 wt % m-phenylene amine and a non-polar solution comprising 0.05 to 0.3 wt % trimesoyl chloride to a surface of a porous support and interfacially polymerizing the monomers to form a thin film polyamide layer, wherein the non-polar solution comprises: a) 50 to 90 vol % of C5 to C20 aliphatic hydrocarbon selected from a paraffin or isoparaffin, or combination thereof, b) 3 to 15 vol % mesitylene, and c) 0.02 to 0.08 wt % 1-carboxy-3,5-dichloroformyl benzene (mhTMC); and ii) applying an aqueous solution of nitrous acid to the thin film polyamide layer.
 2. The method of claim 1 wherein the thin film polyamide layer has a dissociated carboxylic acid content of at least 0.18 moles/kg at pH 9.5 as measured by RBS prior to the step of applying the aqueous solution of nitrous acid.
 3. The method of claim 1 wherein the thin film polyamide layer has a dissociated carboxylic acid content of at least 0.45 moles/kg at pH 9.5 prior to the step of applying the aqueous solution of nitrous acid.
 4. The method of claim 1 wherein the thin film polyamide layer has an isoelectric point (IEP) of less than or equal to 4.3 prior to the step of applying the aqueous solution of nitrous acid. 